(i) Field of the Invention
This invention relates generally to improved dentifrice compositions. More particularly, it relates to toothpaste and related products prepared from both a source of fluoride ions (such as sodium monofluorophosphate) and dicalcium phosphate dihydrate (as a dental polishing agent). Even more particularly, it relates to the avoidance of the use of an excessive amount of trimagnesium phosphate and its hydrates in such toothpaste compositions.
(ii) Related Prior Art
Many dentifrice compositions, e.g., toothpastes, today comprise four or more ingredients essential for effectiveness: (1) a water-soluble fluoride compound (such as sodium monofluorophosphate) as an anti-cariogenic agent; (2) a dispersant comprising water; (3) dicalcium phosphate dihydrate (hereinafter DCPD) as a polishing agent; (4) humectant, such as glycerine or sorbitol; (5) one or a group of several optional stabilizers intended to maintain the essential properties of the toothpaste during storage over a period of possibly years; and (6) optional surfactants.
A long-addressed problem with such prior art toothpastes is their tendency to "age" in several different ways. Firstly, the monofluorophosphate ingredient is converted from a water-soluble form to an insoluble form, apparently by reaction with the dicalcium phosphate dihydrate. This reduces the concentration of the fluoride ions. Secondly, some stabilizers that reduce the reactivity of the dicalcium phosphate dihydrate (thereby preventing it from "setting up" and the toothpaste becoming lumpy), themselves convert the monofluorophosphate into an insoluble form. The foregoing problem has been defined as "monofluorophosphate (or MFP) incompatibility of the DCPD". The problem is further compounded by the fact that the dicalcium phosphate dihydrate raw material itself has to be stabilized against degradation during manufacture and storage prior to use, in a different and non-toothpaste environment.
Prior art relating to the use of a magnesium phosphate as an ingredient in a composition comprising dicalcium phosphate dihydrate includes the following: U.S. Pat. Nos. 4,472,365 (Michel); 4,312,843 (Monty et al.); 4,487,749 (Sherif et al.); 4,496,527 (Sherif et al.); 4,016,255 (Forward et al.); 3,411,876 (Harnisch et al.); and 3,294,786 (Cremer et al.); British Pat. No. 1,548,465 (assigned to Hoechst AG); and the abstract of Japanese Pat. No. 58,052,209-A (assigned to Lion Corp.). Essentially, none of the foregoing discloses the invention claimed hereinafter wherein dicalcium phosphate dihydrate compositions (and toothpastes prepared therefrom) comprise trimagnesium phosphate octahydrate in a critically small range.
Stauffer Chemical Company's U.S. Pat. No. 4,472,365 (Michel) is of particular interest and is hereby incorporated by reference. Michel's invention is mainly directed to the addition of relatively small amounts of magnesium oxide to the reaction mixture during the preparation of dicalcium phosphate dihydrate, in order to improve the "monofluorophosphate compatibility" of the final product. In a preferred embodiment of Michel's invention, dimagnesium phosphate, trimagnesium phosphate or mixtures thereof are optionally blended with the dicalcium phosphate dihydrate in an amount from about 0.5% to about 5% by weight of the dicalcium phosphate dihydrate (see col. 5, lines 22 to 43 and claim 3). They are merely described as being effective to reduce "caking" or "lumping" of the dicalcium phosphate dihydrate and are not described as exerting any effect on the fluoride compatibility of a dicalcium phosphate dihydrate-containing toothpaste in which they might also be present.
There has also been commercial use, for more than one year, of some embodiments of the subject matter of the foregoing Michel patent, with trimagnesium phosphate octahydrate incorporated at the 0.8-1.35 weight percent level.
Stauffer Chemical Company's U.S. Pat. No. 4,312,843 (Monty et al.) relates to dicalcium phosphate dihydrate compositions having improved monofluorophosphate compatibility that are prepared by adding pyrophosphoric acid to the dicalcium phosphate reactor; terminating the reaction by which the dicalcium phosphate is formed at a pH ranging from 4.9 to 5.5; and blending the final product with a stabilizing agent. Monty also discloses that trimagnesium phosphate octahydrate and dimagnesium phosphate trihydrate are both effective as stabilizing agents as judged by monofluorophosphate compatibility. Dimagnesium phosphate was particularly effective, in this regard, at concentrations in the range of from 1 to 5% by weight as giving better fluoride compatibility readings than trimagnesium phosphate.
British Pat. No. 1,548,465 assigned to Hoechst AG relates to the precipitation of dimagnesium phosphate trihydrate either onto or jointly with the precipitation of dicalcium phosphate dihydrate. The product comprises 1-50 (preferably 2-20) weight percent of dimagnesium phosphate trihydrate, based on the dicalcium phosphate dihydrate. The effect of trimagnesium phosphate is not touched upon by this reference.